Diffusionally modified catalysts find use in many shape-selective, hydrocarbon processing applications. The selectivity to more desirable products (and ultimate product slate) can be modified with diffusionally restricted catalysts. Mass transport selectivity arises from a large difference in the diffusivity of the participating molecules in the zeolite channels, white transition state selectivity results from steric constraints limiting the possible transition state of the catalytic transformation step. The advantages of diffusionally modified catalysts are especially useful in certain petroleum/petrochemical industry processes including catalytic dewaxing, olefin alkylation, shape-selective cracking and aromatic conversion processes such as aromatics disproportionation, e.g., toluene disproportionation, aromatics isomerization, e.g., xylene isomenization, and para-selective aromatics alkylation. The optimum level of acidity for these reactions can vary substantially. For selective aromatics disproportionation processes, e.g., toluene disproportionation processes, a high acid level (700 alpha) can produce a high value product slate. Selective ethylbenzene conversion processes are optimized by a medium acidity level (˜50 to 150 alpha), while dewaxing and para-selective aromatics alkylation processes prefer lower acid activities (˜5 to 25 alpha).
Ex-situ selectivated catalysts, such as those modified via multiple silica treatments, are particularly attractive for these processes because the diffusion barrier required for optimal performance is present prior to utilization for the reaction of choice. Presently, high acid activity zeolite catalyst can be used as a base for multiple selectivation sequences, e.g., a 1000 alpha catalyst is used to produce a high acid activity toluene disproportionation catalyst, which after several selectivation treatments still has an alpha value of about 700, while diffusionally modified catalyst for other applications may require lower acid activity as noted above.
Steaming has been used to decrease acid activity of catalysts. However, steaming silica-selectivated catalysts to the lower acid activity levels required for certain applications significantly decreases the diffusional barrier, probably resulting from migration of the silica diffusion barrier during steaming.
Accordingly, it would be desirable to provide a method for modifying zeolite catalyst activity, which does not decrease the diffusion barrier of the resulting catalyst. It would further be desirable to provide a method for modifying zeolites to provide a diffusionally restricted catalyst having reduced acid activity while maintaining or increasing the diffusion barrier of the modified catalyst.
U.S. Pat. No. 5,849,968 to Beck et al. discloses a process for shape-selective hydrocarbon conversion using a zeolite catalyst selectivated with a siliceous material and treated with an aqueous solution comprising alkaline earth metal ions under ion exchange conditions. After selectivation, the zeolite is calcined at temperatures greater than 200° C., including temperatures below 700° C. U.S. Pat. No. 5,610,112 to Lago et al. discloses a process for modifying a catalytic molecular sieve by pre-selectivation to deposit a silicon compound on the external surface of the catalyst and then calcined at a temperature below 600° C. for one to 24 hours. The catalyst may then be steamed at 200° C. to 538° C. to provide improved selectivity. U.S. Pat. No. 5,726,114 to Chang et al. discloses a method for modifying catalytic molecular sieve to enhance shape selectivity by exposing to at least one ex situ selectivation sequence which includes impregnation of the molecular sieve with a selectivating agent in an aqueous emulsion and a subsequent calcination of the impregnated molecular sieve at temperatures below 600° C. U.S. Pat. No. 5,384,296 to Tsao discloses a thermally stable noble metal-containing zeolite catalyst which has increased resistance to noble metal agglomeration as a result of calcining at at least 600° C. in moist air. U.S. Pat. No. 5,034,362 to Chu et al. discloses a zeolite catalyst composition having improved shaped selectivity which has been calcined at a temperature of at least 649° C. which is useful for aromatic conversion reactions. None of these disclosures teach or suggest the use of very high temperature calcination as a means to modify acid activity of selectivated molecular sieves without decreasing diffusional resistance of the modified catalyst.